Yamaha RX-v630 Turns off after 1 second. 1) Removed ALL speakers wire, 2) Check impedance switch on back

SOURCE: Yamaha RX-V995 receiver turns on & then powers off 2 seconds late

I suggest you run self diag.  press [FM/AM] key + [Input mode] key and press the [power/standby] switch.

ps prt = power supply problem
dc prt = shorted output or leakly cap in channel
tmp prt means to hot on heatsink

you'll likely have to measure the resistances of the output FETs to find a short, 

SOURCE: Yamaha RX-V1400 receiver powers off, in case it becomes louder

Hey, yeah that can be frustrating.

When you turn it up do you hear the relay or a clicking sound, then you turn it on and it comes back below the +2db?

Try unplugging All the speaker wires from the unit. Turn on the unit and listen for the click.
after the click, turn it up past the 2+ db mark and see if you hear it click or shut down. If it did not click off, then suspect a shorted crossover on one of your speakers.
2nd test is to plug in One speaker, and test it at high volume, it will not harm it. If it shuts off disconnect that speaker and try another. Only one at a time.
If it works at high volume, you need to isolate one of your speakers and i bet it has a shorted crossover that has been cooked. P.S. If one is bad, suspect they will fail at a later date.
If they all fail, use a friends speaker and try it before sending it to a repair shop.

P.S. i found one on ebay i think you can get for under $100 bucks in the box.

http://cgi.ebay.com/Yamaha-RX-V1400-7-1-THX-Select-Receiver-MINT-Condition_W0QQitemZ330311512027QQcmdZViewItemQQptZReceivers_Tuners?hash=item330311512027&_trksid=p3286.c0.m14&_trkparms=72%3A570|66%3A2|65%3A12|39%3A2|240%3A1318

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SOURCE: Yamaha RX-V595a receiver shuts off 2 seconds after its powered on

The first thing called out is the impedance switch on the back is not fully in one or the other position. Cycle the switch back and forth to make sure it will 'read' each setting, then turn it on.

Also make sure you if you have anything connected to its convenience outlets that they are not drawing more current than is allowed.

Did you buy it used or new?